1. Every programming language has its specific syntax. You cannot simply write mathematical expression, it is required to use some symbols called operators.
2. You have to assign value to xbefore you evaluate polynomial.

instead of polynomial=3x^3-5x^2+6;
try using the "pow function" polynomial=3*pow(x,( 3))-5*pow(x,(2))+6;
You could declare x as const int x=?; above your polynomial variable;
logically it could work, but id google the pow function to make sure i wrote it right
its been a while since ive used it.